The word "radome" dates back to World War II and comes from the two words `radar` and `dome`. Originally, radome referred to radar transparent, dome-shaped structures used to protect radar antennas on aircraft. Over time, however, radome has come to mean almost any structure that protects and serves as a "window" for electromagnetic radiation up to 1,000 GHz. Such structures may be ground based and may be flat rather than dome shaped. Commercial aircraft carriers typically utilize a nose radome. Accordingly, for purposes of brevity, the term radome will hereinafter be understood to refer to a nose radome installed on aircraft to protect weather radar.
The need for all-weather operation of both military and commercial aircraft demands that an effective weather tracking radar be operational at all times. A device is needed to cover the antenna that is strong enough to protect it, yet transparent to radar.
A radome is an integral part of a radar system because thickness and other properties affect the effectiveness of the radar set. This requires that a radome be compatible to the specific properties of the radar set used therein. Major design criteria of a radome include: radar transparency, structural integrity, aerodynamic shape, and light weight. Economics also require that the cost should be as low as possible and the service life as long as possible. Successful radome design must balance all of the conflicting requirements. For example, the ideal shape of a nose radome from an electrical standpoint is hemispherical and as large as the aircraft will allow. A better aerodynamic shape, however, is conical. A thick wall would have structural benefits, yet for optimum radar transmission, the wall must be a factor of the wavelength. A lightweight design may improve aircraft performance, save fuel, and occasionally reduce product cost at the expense of decreased service life, increased maintenance costs, and/or increased product costs.
It is well known in the art that radomes fail when subjected to severe structural damage or radar degradation. There are numerous ways for this to occur in the hostile environment in which radomes must operate. High velocity rain is widely recognized as a leading cause of radome failure. Impact and erosion due to rain initiate damage and pinholes. Additionally, rain causes further damage as it seeks pinholes (i.e. moisture paths) and penetrates into the core.
Moreover, high velocity rain impacts and erodes paint systems and radome skins, particularly in the forward area of the radome. This opens moisture paths and reduces structural integrity. Solutions to the rain erosion problem include maintaining a rain erosion resistant painting system on the radome. Polyurethane and rubber boots are also available to install on the tip of radomes. If adequate erosion protection is used, the dominant mode of failure due to high velocity rain appears to be core impact failure or "soft spots." This promotes microcracking and moisture propagation.
It is also well known in the art that moisture, in the form of water and/or ice, can enter an open-cell core through any microcrack or pinhole in the skin. Altitude changes lead to freeze-thaw cycles, which causes water to expand by about 10% when it freezes. Repetitive freezing and thawing results in delaminations, cracking and the like in the core. Water and ice are also detrimental to radar transmissivity as their dielectric constant is on the order of 20 times greater than that of most materials used for sandwich construction nose radomes.
Additionally, multiple impact strikes are sustained during rain and hail storms. Bird and equipment strikes can inflict major single impacts. Smaller impacts can damage the radome's outer surface, causing delaminations, microcracks and opening up moisture paths. A large enough bird or equipment strike could go completely through the radome and severely damage the antenna.
Moreover, lightning strikes also pose serious problems to radomes. Depending on the current in the strike, lightning can penetrate the radome and damage the antenna, delaminate large sections or leave a microscopic pinhole. Even small holes and delaminated areas open moisture paths. Therefore, many radomes are equipped with lightning protection. This usually consists of strips of conductive material that are grounded to the fuselage. The strips must be placed so that the radar transparency of the radome is not adversely affected. While this does not alleviate the problem entirely, lightning diverter strips do reduce the risk of damage by conducting lightning to the fuselage and away from the antenna and radome.
Finally, static electricity on the outer surface of a radome can burn through the wall when the charge moves towards the antenna or another electrically conductive surface. Static burns are small, about the size of a pinhole, and the surrounding area may be blackened. Even so, any puncture allows moisture into the structure. This can be avoided by using anti-static paint or primer, which permits static electricity to bleed off to the airplane before a charge large enough to create a hole can build up.
Currently, the most common radomes among subsonic and transonic aircraft are fiberglass reinforced honeycomb core sandwich construction radomes. Standard hexagonal cell shaped honeycomb is generally not flexible enough for tight radii. Therefore, in many radomes, a higher density honeycomb variation called "flex core" is used in the nose section.
Honeycomb core radomes have excellent static strength/stiffness-to-weight ratios, excellent radar transparency, and are relatively easy to process. However, honeycomb core has an open-cell structure which encourages moisture intrusion, and it has relatively poor impact properties. Some honeycomb core radomes include a layer of polyvinyl fluoride (TEDLAR.RTM.) on the inside skin to aid in sealing out moisture.
Static properties, FEM analysis and testing traditionally have led aircraft designers to select honeycomb core to construct the "best" radome. Although "best" is often defined as lightest, stiffest and strongest, this approach is often inadequate, especially in impact/moisture critical environments, such as radome and marine applications. The FAA repair station has collected radome repair data for about 20 years. About 85% of all honeycomb radomes are removed for moisture, and most air carriers confirm that their mean-time-between failures is substantially less than two years for "737" style honeycomb radomes. Consequently, high maintenance costs, high inventory and questionable radar performance (due to moisture) occur.
As noted above, some of the numerous ways for moisture paths to be created are impacts from hail, rain, bird, equipment strikes, static electricity pinholes and stress microcracks, which may be invisible. During flight, however, dynamic wind pressure pumps water through the microcracks into the core.
Once moisture gets through the skin, it collects in the honeycomb cells of prior art radomes. As the water freezes, it expands and cracks the cell walls leaving a path for moisture contamination to propagate. Over time, a large portion of the radome may be damaged from just one crack. Even if structural damage is not evident, moisture contamination must be repaired because the presence of water and ice severely diminish radar performance.
Another common type of radome used in aircrafts are the fluted core radomes, which are manufactured for McDonnell Douglas radomes. Fluted core is a series of square fiberglass tubes and was adopted to combat the moisture contamination problem associated with the honeycomb core radomes. Ideally, any moisture introduced into the radome flows through the flutes away from the electrically critical window. The moisture resistance of this type of core is somewhat better than that of honeycomb, thereby providing longer service life and fewer repairs.
However, fluted core has a high density (approximately 200 kg/m.sup.3), which is over twice as dense as other radome core materials. In addition, the construction of a fluted core radome is very labor intensive, which leads to an expensive finished product. Furthermore, repairs are expensive and time consuming. A fluted core radome also weighs approximately 30% more than its honeycomb counterpart. The weight and expense trade-offs are not acceptable to many radome designers, especially since fluted core radomes eventually retain moisture.
Yet another type of radome known in the art is the foam core radome. Foam-in-place radomes (polyurethane foam) were popular in the 1950's, but its tendency to crumble, poor fatigue and poor impact properties quickly gave "foam radomes" an unfavorable name. Other foams that are touted as closed-cell (i.e. polymethacrylimide foam) actually have poor moisture absorption properties. This history of poor "foam radome" performance has hindered the development of other radomes using a better suited foam.
There has therefore been a long-felt need to provide a radome construction that solves the longstanding problems of the prior art.